1. Field of the Invention
This invention relates to a throttle device for an internal-combustion engine and, more particularly, to an electronically controlled throttle device which controls the opening and closing operation of a throttle valve by driving an electric actuator according to a control signal.
2. Description of Related Art
In an electronically controlled throttle device which controls an engine throttle valve by driving an electric actuator (e.g., a D C motor and a stepping motor), there has been known such a technology that the amount of initial opening (default opening) of the throttle valve is set larger than a full-closed position when an ignition switch is in off position (in other words, when no current is being supplied to the electric actuator).
Here, the full-closed position of the throttle valve is not meant by a position in which the intake air passage is full-closed; especially in a throttle device having no bypass around the throttle valve and controlling the idling speed only by means of the throttle valve, the full-closed position is defined as a mechanically full-closed position and an electrically full-closed position which will be described below.
The mechanically full-closed position is the minimum opening position of the throttle valve defined by a stopper. The minimum opening is set at a position where the intake air passage is slightly opened from a full-closed position to thereby prevent the throttle valve from galling. The electrically full-closed position is the minimum opening position within the range of opening used in engine control, and is set, by the control of the electric actuator, at a position of a slightly wider opening than the mechanically full-closed position (e.g., about 1 deg. larger than the mechanically full-closed position).
In the electronically controlled throttle, the electrically full-closed position (the minimum opening for control) and the idle opening (an opening required for controlling the idle speed) do not necessarily agree. This is because the amount of opening of the throttle valve is controlled by a feed-back control system according to an idle speed detection signal in order to keep a target idle speed, and for this purpose the amount of opening is allowed to vary.
The full-open position has also a mechanically full-open position defined by the stopper and an electrically full-open position in which the throttle valve is opened to the maximum control amount of opening. The full-closed position stated herein includes the mechanically full-closed position and the electrically full-closed position as well. In normal control, the throttle valve is controlled within the range from the electrically full-closed position (the minimum opening for control) to the electrically full-open position (the maximum opening for control), so that a part of the throttle valve shaft will not hit on the stopper which determines the mechanically full-closed and full-open positions, when the throttle valve is being controlled to the minimum or maximum opening. Thus it becomes possible to protect the stopper and throttle components from mechanical fatigue, abrasion, and damage, and also to prevent galling to the stopper.
The default opening (i.e., the initial opening when the ignition switch is in off position) is set to the amount of opening of the throttle valve which is opened wider than the full-closed position (the mechanically full-closed position and the electrically full-closed position) (e.g., 4 to 13 deg. wider than the mechanically full-closed position).
The default opening is set from the reason for achieving the air flow rate necessary for fuel combustion for operation to be performed prior to engine warm-up at the time of engine starting (cold starting) without providing an auxiliary air passage (an air passage bypassing the throttle valve). During idling, the throttle valve is controlled towards decreasing the amount of opening from the default opening as the engine warm-up proceeds (in this case, however, the electrically full-closed position is the lower limit position).
Furthermore, the default opening is adopted to meet requirements for insuring self-running (limping home) in the event of a throttle control system trouble or insuring an intake air flow rate necessary for preventing an engine stop, and for preventing the throttle valve from being stuck with a viscous substance, ice, or other, on the inside wall of the throttle body.
As a conventional example of a default opening setting mechanism, various mechanisms have been proposed. A known prior art has been stated in, for example, Japanese Laid-Open No. Sho 63-150449 Patent Publication, U.S. Pat. No. 4,947,815 specification , Japanese Translation of PCT Application No. Hei 2-500677 corresponding to the U.S. patent, Japanese Laid Open No. Sho 62-82238 Patent Publication and its corresponding U.S. Pat. No. 4,735,179 specification, Japanese Laid-Open No. Hei 10-89096 Patent Publication, and Japanese Laid Open No. Hei 10-131771 Patent Publication.
There are various types of default opening setting mechanisms, a typical type of which for example is as follows.
One type is of such a system that a default opening setting engagement element (a default lever) which is fitted on the throttle valve to enable the rotation of the engagement element on the throttle valve shaft is engaged via a spring with an element secured on the throttle valve, thereby allowing the default lever to turn together with the throttle valve shaft between the range from the default opening position to the valve full-open position. When the ignition switch is in off position, the default lever is held in contact with the default stopper, to thereby hold the throttle valve opening at the default opening. To close the throttle valve to the default opening or less, the default lever is disengaged from the throttle valve shaft to allow the throttle valve shaft to rotate independently against a spring force towards closing the throttle valve.
Another type is of such a system that, reversely to the above-described system, the default lever and the throttle valve shaft are turned together from the throttle valve full-close position to the default opening position. When the ignition switch is off, the default lever is held in contact with the default stopper to hold the throttle valve opening at the default opening. When the throttle valve is opened over the throttle opening, the default lever is disengaged from the throttle valve shaft, to allow the throttle valve shaft to turn towards opening independently against the spring force.
The electronically controlled throttle device can perform more accurately the air flow rate control suitable for the operation of the internal-combustion engine than a mechanical throttle device which transmits the amount of depression of the accelerator pedal to the throttle valve shaft through an accelerator cable. The component count is increased to provide an electric actuator, a default opening setting mechanism, and a throttle sensor. Therefore, downsizing, weight reduction and simplification, rationalization of fabrication and adjustment jobs, and further improvement in operation stability and accuracy of the throttle body, are demanded.